Further progress is reported herein in our studies of evolutionary and genetic studies with Drosophila as the test organism. Our investigations on the mechanism of DNA repair and replication continue with the isolaton of new mutagen sensitive (mus) mutants on the X chromosome, and the mapping of X chromosome mutants to 12 complementation groups. Third chromosome mus mutants fall into a minimum of 12 complementation groups with one mutant, mus(3)307D2, exhibiting potent effects on chromosome disjunction and crossing over. Development of biochemical assays designed to define the biochemical lesions associated with the mus continues with two mutants mei(1)9 and mus(2)201 being excision deficient. Studies of Drosophila populations include assessment of the frequency of MR (mutation recombination) chromosomes in D. melanogaster, the production of deletions by the MR chromosomes, fitness interactions between chromosomes isolated from wild flies, temporal changes in allozyme frequencies in D. pseudoobscura and D. persimilis; fitness of allozyme variants in D. pseudoobscura; natural history studies of Hawaiian Drosophila species and a biochemical definition of alcohol dehydrogenase allozymes of D. melanogaster.